1. Field of the Invention
The present invention relates to a low-cost method and instrument for performing non-invasive blood glucose measurements.
2. Description of the Background Art
People with diabetes need knowledge of their blood glucose level in order to determine their medication dosage including the use of insulin. This has led to a large market for invasive and minimally invasive instruments. Such instruments require drawing a small sample of blood, either from the fingertip or other part of the body (e.g., forearm, thigh, etc.). The sample is placed on a chemically treated disposable strip which is inserted into a small battery powered instrument to determine blood glucose level in the body.
There are two disadvantages of the invasive and minimally invasive instruments. First, there is the pain and discomfort of making a measurement, particularly at a fingertip site, and second, the disposable test strips are costly (e.g., 70¢ per strip).
Although the minimally invasive instruments may eliminate a majority of the discomfort associated with making a measurement, the glucose level of interstitial fluid that is drawn at sites away from the fingertips may be significantly different from the true blood glucose level as there is a significant time delay before the glucose level of the interstitial fluid adjusts to the actual blood glucose level. This causes potentially serious, and even medically-threatening, measurement errors. In particular, such errors can be significant when the glucose is rapidly falling into the hypoglycemic range, as disclosed in “Glucose Monitoring at the Arm: Risky Delay of Hypoglycemic and Hyperglycemia Detection,” Jungheim, K. et al., Diabetes Care, Vol. 25, No. 6, pp. 956–60 (June 2002), which is incorporated herein by reference.
Further, interstitial fluid instruments can be in error by as much as 100 to 150 mg/dL compared to true blood glucose in the first ninety minutes following the consumption of a meal, as disclosed in “Rapid Changes in Postprandial Blood Glucose Concentration Differences at Finger, Forearm, and Thigh Sampling Sites,” Ellison, J. M. et al., Diabetes Care, Vol. 25, No. 6, pp. 961–64 (June 2002), which is incorporated herein by reference. Even if the above errors were tolerable, the test strips for the minimally invasive instruments cost approximately the same as the test strips for the invasive (fingertip) instruments.
Non-invasive instruments for measuring blood glucose levels are desirable and are well known in the art. For example, U.S. Pat. No. 5,077,476 issued on Dec. 31, 1991 to Rosenthal and U.S. Pat. No. 5,086,229 issued on Feb. 4, 1992 to Rosenthal, both incorporated herein by reference in their entirety, use near-infrared quantitative technology for making such measurements. Both patents disclose using a near-infrared energy source (e.g., IRED) for introducing near-infrared energy into a test subject's finger.
One limiting aspect of the near-infrared quantitative non-invasive measurement of blood glucose is that it requires a very precise optical measurement at multiple wavelengths (e.g., eight or more wavelengths). The required precision and the large number of required measurement wavelengths makes such an instrument complex and potentially costly.
Another limiting aspect is that the calibration system performs a multiple regression analysis to calculate the calibration constants. Although the multiple linear regression analysis has great value, it requires a large number of calibration samples, and it limits subsequent glucose measurements to the range of calibration samples.
Thus, there is a need for a low cost non-invasive method and apparatus, which requires a small number of calibration samples, for measuring the blood glucose concentration in blood.